# std::indirect_unary_predicate

From cppreference.com
< cpp‎ | iterator

Defined in header `<iterator>` |  |
template< class F, class I > concept indirect_unary_predicate = std::indirectly_readable<I> && std::copy_constructible<F> && std::predicate<F&, /*indirect-value-t*/<I>> && std::predicate<F&, std::iter_reference_t<I>>; |  | (since C++20)

The concept `indirect_unary_predicate` specifies requirements for algorithms that call unary predicates as their arguments. The key difference between this concept and std::predicate is that it is applied to the type that `I` references, rather than `I` itself.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P2609R3 | C++20 | one of the requirements was defined in terms of std::iter_value_t<I>& which mishandled projections resulting in incompatibility with predicate F& | defined in terms of /*indirect-value-t*/<I> to correctly handle such projections
P2997R1 | C++20 | `indirect_unary_predicate` required F& to satisfy `predicate` with std::iter_common_reference_t<I> | does not require

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/iterator/indirect_unary_predicate&oldid=175478"